GB2077500A

GB2077500A - Thermal fuse

Info

Publication number: GB2077500A
Application number: GB8015826A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-05-13
Filing date: 1980-05-13
Publication date: 1981-12-16

Abstract

A thermal fuse comprises a contact spring 3 which bridges two contact terminals 1,2, at least one of the contact terminals 2 being secured to the contact spring 3 by fusible material so that the contact spring 3 separates from the contact terminal 2 when the fusible material melts at a threshold temperature, whereby to break the electrical path interconnecting the contact terminals 1,2 through the contact spring 3. In the manufacture of the fuse, the contact spring 3 is maintained in an unstressed condition during its connection to the contact terminals 2, which are then moved to stress the spring 3. The contact spring 3 may be secured at both ends to the respective contact terminals 2 by fusible materials having different melting temperatures. <IMAGE>

Description

SPECIFICATION Thermal fuse This invention relates to a thermal fuse for opening an electrical circuit at a threshold temperature in order to prevent overheating of equipment incorporating the circuit.

It is known to provide electrical equipment with mechanically acting, irreversible thermal fuses to protect the equipment from overheating through equipment malfunction, operator misuse or overvoltage leading to increased heat loss and therefore increased temperature. Such fuses have to be replaced by new fuses by a service or repair engineer before the equipment will function again, preferably checking the reason for the original fuse being blown. The safety advantages of such fuses are being increasingly recognised by electrical authorities in numerous countries.

One type of known mechanically acting, irreversi blethermal fuse comprises a pellet which holds an electrical contact element in position, against the biassing force of a spring, to complete an electrical circuit. If the local temperature increases to a threshold level, the pellet melts and the biassing spring moves the contact element so as to break the electrical circuit. In another type of known fuse, an electrical contact element bridges two contacts by being secured to these contacts by a fusible alloy, and a biassing spring acts on the contact element in such a direction as to separate the contact element from the contacts when, at a threshold temperature, the fusible alloy melts.

in accordance with this invention, there is provided a thermal fuse comprising a contact element connected to two contacts so as to bridge those contacts, at least one of the contacts being secured to the contact element by material having a threshold melting temperature, and the contact element having a spring characteristic and arranged to bias the contact element to separate from said one contact upon said securement material melting at the threshold temperature.

This fuse exhibits a simple mechanical design with less parts involved in its operation then the aforementioned prior art fuses (eliminating the separate spring), thus minimising production and assembly and leading to higher reliability and lower unit costs.

The fuse may respond to the ambient temperature exceeding the threshold temperature, and/or it may respond to an excessive level of current flowing through it, have the effect of heating and melting the material securing the contact element to the at least one contact.

Embodiments ofthis invention will now be described, by way of examples only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a cross-section through a fuse in accordance with this invention, also showing its spring contact element, at an intermediate stage of production, secured to only one of the two contacts of the fuse; Figure 2 is a cross-section through a second embodiment of fuse; Figure 3 is a diagram of the contacts and spring contact element of a third embodiment of fuse showing in Figure 3a the disposition of the parts for securing together and in Figure 3b the final disposition of the parts; and Figure 3 is a diagram of a modification for using, for securing the spring element to the contacts, adhesive or resin which is not electrically conductive.

Referring to Figure 1, the fuse comprises two contact posts 1, 2 disposed parallel to each other and having their ends bridged by an elongate spring contact element 3 which is secured at its opposite ends to said ends of the contact posts. The securement of the contact element 3 to at least one of the contact posts is effected by material having a threshold melting temperature. The contact element has a spring characteristic and is arranged under stress such that it will separate from said at least one contact post upon that securement material melting at the threshold temperature, thus opening the electrical circuit through post 1, contact element 3 and post 2. Contact element 3 may comprise a copper alloy of high electrical conductivity and having a spring characteristic.Posts 1 and 2 are mounted in an insulating base 7 through which they project, and enclosed within a metal or plastics protective housing 8.

In a simple method of production, contact element 3 may be hand soldered to post 1 by means of a low melting temperature metal alloy 4, with the spring element 3 in its natural or unstressed condition.

Then the other end of the spring element 3 is moved into contact with the other post 2, thus stressing the spring element, and now either soldered (with a metal alloy of melting temperature higher or equal to that securing element 3 to post 1) or resistancewelded to post 2. If the posts are soldered to the spring element by alloys of different melting temperatures, the fuse could have two triggering levels with a high probability that at least one of the connections will break with increasing temperature.

An automated production could for example comprise the following steps. Firstly, the post 1 and spring element 3 are separately alloyed in a bath.

Then the post 1 and spring element 3 are held by respective electrodes and laid to abut each other in the required position (with the spring element 3 in its natural, unstressed state). Then a high current surge is passed by way of the electrodes to melt the alloy and provide the required securement of the spring element 3 to the post 1 upon cooling. Then the other end of the spring element 3 is abutted against the end of post 2, with post 2 positioned to place no stress on the spring element 3. The post 2 is then secured to the spring element by resistance or spot welding. The posts 1 and 2 are next moved to the parallel relation shown to stress the spring 3, in a jig for receiving the assembly, and the insulating base 7 and housing 8 are applied.

Figure 2 shows an embodiment of fuse in which the posts 1 and 2 are in an in-line or axial disposition.

One end of the spring element 3 is welded to post 2 and its other end is secured by metal alloy 4 to post 1, the spring element separating from post 1 in the direction of the arrow when the metal alloy 4 melts if the threshold temperature is reached. The assembly procedures may correspond generally to those described above.

Figure 3 shows an embodiment of fuse in which the posts 1 and 2 are in the in-line disposition. Firstly, the ends of the S-shaped spring element 3 are secured to the ends of the posts 1 and 2, at opposite sides thereof, which posts are off-set (Figure 3a) to avoid stressing the spring element. Then the posts are moved to the in-line position (Figure 3b) to stress or tension the spring. The ends of the spring element will separate from the respective posts in the directions shown by the arrows.

Instead of a low melting point alloy, a low melting point adhesive or resin material may be used. If the material is not sufficiently conductive to pass electrical current, then electrical contact between the contact post and the spring contact element may be provided at a separate location. Figure 4 demonstrates such an arrangement, in which the spring element 3 makes electrical contact with the post 2 at the corners 10 of an enlarged diameter end 9 of the post: the contact points of the spring element 3 lie inwards from the pointofsecurementofthe element 3 to post 2 by the adhesive or resin 4 and ensure good electrical contact by the local high pressure on the corners of the enlarged end 9 of the post.

Claims

1. Athermal fuse, comprising a contact element connected to two contacts so as to bridge those contacts, at least one of the contacts being secured to the contact element by material having a threshold melting temperature, and the contact element having a spring characteristic and arranged to bias the contact element to separate from said one contact upon said securement material melting at the threshold temperature, whereby to break an electrical path interconncting the two contacts through the contact element.

2. A thermal fuse as claimed in claim 1, in which the contact element is secured to the respective contacts by securement materials having different melting temperatures, the contact element being biased to separate from either contact upon melting of the respective securement material.

3. A thermal fuse as claimed in claim 1 or 2, in which the two contacts comprise two contact posts arranged in an in-line disposition with the contact element being being secured at one end to one side of one contact post and at its other end to an opposite side of the other contact post.

4. Athermal fuse as claimed in claim 1 or 2, in which the two contacts comprise respective contact posts disposed generally parallel, with the contact element being secured to adjacent ends of the respective contact posts.

5. Athermal fuse as claimed in any preceding claim, in which the securement material is electrically non-conductive or of a relatively poor electrical conductivity, and the contacts are formed with projections making electrical contact with the contact element.

6. A method of forming a thermal fuse, comprising connecting a contact element, which has a spring characteristic, to two contacts so as to bridge those contacts, a material having a threshold melting temperature being used to secure at least one of the contacts to the contact element and the contact element being maintained in a disposition in which it is substantially unstressed during its connection to the contacts, whereafter the contacts are moved relative to each other to stress the contact element and impart a bias to itto separate from said one contact upon said securement material melting at the threshold temperature.

7. A method as claimed in claim 6, in which firstly an alloy coating is applied to the contact element and said one contact, then the contact element is positioned to abut said one contact and an electrical current is passed through the contact element and said one contact to melt the alloy and effect securement upon cooling.

8. A thermal fuse substantially as herein described with reference to Figure 1,2,3 or 4 of the accompanying drawings.

9. A method of forming a thermal fuse, said method being as claimed in claim 6 and substantially as herein described.